This invention relates to optical encoders having patterns of encoding illuminations and more particularly to such optical encoders including an improved light distribution system having an efficiently arranged pattern of discrete output couplers for producing the encoding illuminations therein.
In optical encoders generally, and particularly in certain shaft angle or shaft position optical encoders, coded electrical or electronic signals are generated at photosensitive sensors in response to variations in encoding illuminations. Typically, an array of several photosensitive sensors has a predetermined pattern so that each sensor is selectively responsive to separate encoding illuminations. An associated shaft angle code pattern is usually rotated by the shaft to be monitored so that the code pattern is rotated between the sensor array and the source of the illuminations. Opaque and transparent segments of the code pattern selectively block and transmit the separate encoding illuminations to the sensors. Thus, the sensor array will provide different optically activated states correspondingly representative of the different rotated shaft positions to be encoded.
In the above type of encoder, the array of photosensitive sensors is selectively responsive to the separate encoding illuminations wherein each has a predetermined radiation level corresponding to a predetermined quantity of light energy required for effecting the desired activated response at each sensor. The source of encoding illuminations is often provided by a light distribution system including a light guide plate having discrete light output couplers for directing individual output radiation beams to separate sensors of the array. The light guide plate typically conducts a predetermined amount of radiation that is available for producing the output beams at the predetermined radiation levels. The radiations can be propagated in different light distributing modes within the light guide plate depending upon the manner in which the output beams are to exit from the output couplers. The amount of radiations and the mode in which the radiations are propagated in the light guide plate are often determined by an input coupler used to collect input radiations into the light guide plate. An electrical or electronic light source typically produces the input radiations which are incident on the input coupler. These light sources have predetermined capacities and light emitting characteristics and further have given input power ratings for corresponding light emitting intensities. Thus, the overall efficiency and operative effectiveness of a light distribution system can be characterized by comparison of the input electric power of an electrical or electronic light source and its radiant power output relative to the source radiations which are effective in being collected and conducted by the light guide plate so as to be exited in a predetermined number of output beams producing the predetermined individual radiation levels thereof.
Examples of optical encoders of the general type noted above are disclosed for encoding dial readings of utility meters in U.S. Pat. Nos. 4,037,219 issued July 19, 1977, and 4,137,451 issued Jan. 30, 1979, both assigned to the assignee of this invention, and also in U.S. Pat. No. 3,846,788 issued Nov. 5, 1974. One form of the present invention is an improvement of the optical encoders disclosed in the two aforementioned U.S. Pat. Nos. 4,037,219 and 4,137,451. The optical encoders described in the aforementioned patents are included in meter registers for encoding the meter reading indications of at least five dial pointer shafts. Encoding illuminations are provided around each of the dial shafts in a closely spaced and compact arrangement due to the closely spaced relationship of a photosensitive sensor array and compact assembly of shafts and other parts of a dial register mechanism. In the first two of the above noted patents, a light distribution system has a light guide plate for producing the encoding illuminations from patterns of reflecting conical recesses. A single incandescent lamp light source, requiring one hundred milliamperes at twelve volts (one and two-tenths watt) in one embodiment, produces input incident radiations that are widely dispersed to be collected by an optical input of the light guide plate formed by one narrow plate edge. The light conducted by the light guide plate is required to be essentially perfectly diffused therein which is sometimes difficult to provide even with larger-power lamp sources. The conical output reflectors are intended to reflect diffused radiations conducted therein, however, the lack of complete diffusion produces uneven output beams. Thus, the output beams are not highly directional nor collimated which reduces their efficiency in illuminating the sensors at required radiation levels to effect a desired response uniformly at each sensor. The total emissions of lamp source are somewhat inefficiently collected and then not precisely distributed relative to the number and levels of effective output beam radiations required. The latter U.S. Pat. No. 3,846,788 discloses a flat electroluminescent panel covered by a light mask substantially covering the panel except for slots or holes therein defining individual light spots or small light exiting sources for furnishing the encoding illuminations to an array of photosensitive sensors.
It is generally desired to eliminate the use of a relatively large and higher power light source for the encoder light distribution systems as described above having an incandescent lamp source. It is also desired to eliminate the heating effects of incandescent lamp source, and reduce the large amount extraneous light emissions in the encoders due to the wide dispersion of the lamp emissions. Also, it is desired to avoid a separate large power supply required for supplying an incandescent lamp source and to avoid loss of operation due to lamp filament failures by replacing the lamp sources with alternative smaller and lower power light sources such as provided by some solid state light emitting sources including the semiconductor light emitting diode (LED) type. Accordingly, to utilize the alternative light sources, the efficiencies of the prior light distribution systems must be substantially improved.
Accordingly, it is a general requirement of the present invention to provide general improvements in a light distribution system for optical encoders including a light guide plate that is adapted to utilize low power light sources for more efficiently producing a large number of encoding illuminations to a photosensitive sensor array. To provide improved efficiency in the light guide plate, output beams forming the encoding illuminations must each produce a predetermined radiation level in an optically isolated and highly directional beam configuration effective to optically activate each sensor at a desired level of sensor response. The amount and mode of conducting internal radiation beams within the light guide plate must be carefully controlled and maintained to be made substantially wholly available to be intercepted at output couplers to produce precisely defined output beams each having the predetermined level of radiation therein. Also, the light input coupler to the light guide plate must collect a maximum amount of incident input radiations from the available light emitted from the low power light source. The collected incident radiations must be applied to the light guide plate for conducting the internal radiation beams therein for maximum use at the output couplers.
With the foregoing requirements and desired improvements in mind for a more efficient light distribution system for producing encoding illuminations in an optical encoder, the present invention is made as briefly summarized hereinafter.